Field of the Invention
The invention relates to an axle driving apparatus designed to drive a zero turn radius vehicle.
Background Art
An axle driving apparatus having a hydrostatic transmission (HSTs) is generally known in the art. An HST includes a center section on which is mounted a hydraulic pump and a hydraulic motor. The hydraulic pump and the hydraulic motor each carry a plurality of reciprocating pistons that are in fluid communication through porting formed in the center section. As the hydraulic pump rotates, the pump pistons move axially as they bear against an adjustable swash plate where the degree of axial movement depends upon the angular orientation of the swash plate. Axial movement of the pump pistons forces the hydraulic fluid through the porting, which forces the motor pistons against a thrust bearing to thereby rotate the hydraulic motor. As the hydraulic motor rotates, hydraulic fluid is returned to the hydraulic pump through the porting. In this manner, the rotation of the hydraulic pump is translated to the hydraulic motor and the rotation of the hydraulic motor may be used to drive an axle or a pair of axles through a differential. In the case of a transaxle, both left and right wheels are driven together so the speed cannot be independently controlled. This results in a larger turning radius, making it difficult to make close turns. When used in lawn or garden tractors, it is difficult to move close to trees or other obstacles.
Alternatively, there can be one HST for each wheel so that each wheel can be driven independently. Such zero turn HSTs provide for independent control of each of the drive wheels. Improvements and modifications are frequently being made to HSTs with a zero turn radius.